1. Field of the Invention
The present invention relates to a device for modulating and reflecting incident light, and more specifically to a multi-layer device with at least one layer formed of an electro-optic material having a variable index of refraction.
2. Description of Related Art
A typical reflecting device is a first-surface mirror formed of a substrate coated with a metal film or other reflective material. FIG. 1 shows such a device, including a substrate 2 having on its upper surface a coating of metal film. The metal film is typically formed with a layer thickness of greater than 5 microns. First surface metal mirrors generally exhibit a high reflectivity over a broad band spectrum of incident light.
Another conventional mirror configuration is a dielectric stack 6 formed above the substrate 2. As shown in FIG. 2, the dielectric stack may consist of three layers, layers L1 and L3 having a high index of refraction and layer L2 having a low index of refraction. Each layer, L1, L2 and L3, has a thickness of one-quarter wavelength.
As shown in FIG. 2, the change in index of refraction between the layers in the dielectric stack results in the reflection of the incident light. Layers L1 and L3 could thus obviously be formed of a material with a low index of refraction, and layer L2 could be formed of a material with a high index of refraction. Similarly, the stack can have any odd number of layers, e.g., three, five, seven or nine, and may be formed of multiple cells (stacks). In any event, however, because the one-quarter wavelength thickness of the layers is based on a certain wavelength of incident light, a dielectric mirror tends to have a very narrow spectral range of reflectance. Moreover, a dielectric mirror is "static" in the sense that it has no capability to modulate reflected light, nor can it be used to transmit a portion of the incident light and reflect the remaining portion.
U.S. Pat. No. 4,054,362 to Baues, issued Oct. 18, 1977 discloses a device for modulating polarized light passing through an electro-optic material having a variable index of refraction. A plurality of strip electrodes are disposed on the electro-optic layer with adjacent strips connected to opposite poles of a voltage source to form an interdigital electrode structure. The application of a modulated voltage to the interdigital structure creates electric fields in the electro-optic material extending parallel to the direction of propagation of light in the material to create changes in the index of refraction of the material.
In the absence of an electric field applied to the electrode structure, the electro-optic material acts as a waveguide, and the incident polarized light passes through the material. When a voltage is applied to the electrodes, electric fields are produced in successive opposite orientations in the material beneath the electrodes and extend parallel to the propagation of light. The fields change the index of refraction of the material and cause multiple reflections of the light, causing light interference if appropriate space between electrodes is employed. Thus, the light transmitted through the crystal can be varied between a maximum intensity and a total extinction.
The structure and operation of the device described above limits its use to the modulation of only polarized incident light. Also, like the dielectric mirror, the device cannot be used to transmit a portion of the incident light and reflect the remaining portion.